1. Field of the Invention
The present invention relates to a spread illuminating apparatus for a liquid crystal display, and more particularly to a spread illuminating apparatus adapted for irradiating light emitted from a spot-like light source on a screen of a liquid crystal display.
2. Description of the Related Art
In recent years, a spread illuminating apparatus of side light system, which has a light source disposed on a side surface of its light conductive plate, has been often used as an auxiliary apparatus for a light crystal display (hereinafter referred to as LCD). In this type of spread illuminating apparatus, a linear light source, such as a fluorescent lamp, having a length substantially equal to the width of the light conductive plate is used as a light source thereby uniformly illuminating the entire light conductive plate.
In an apparatus, such as a cellular phone, which requires reduced power consumption, a light emitting diode (hereinafter referred to as LED) must be used as a light source. However, when the LED is used, there arises a question about luminance uniformity over the light conductive plate. The question is addressed as described below, and uniformity with no practical problem is achieved (refer to Japanese Patent Publication No. H10-293202). The light entrance surface of a light conductive plate, which has LEDs disposed thereat, is serrated thus forming a series of prisms in order to spread light emitted from the LEDs in the across-the-width direction of the light conductive plate. The question is also addressed such that an optical path conversion means comprising, for example, a dotty roughened surface is provided on the bottom surface of the light conductive plate to thereby allow light introduced into the light conductive plate through the entrance surface to exit out through the exit surface. And, in some cases, a diffuser plate may be provided between the light conductive plate and the LCD to further improve the uniformity.
Since not only reduction in power consumption but also a higher luminance is increasingly required in the spread illuminating apparatus, there is a demand for an efficient use of light emitted from the LEDs. To meet the demand, it is considered to narrow the view field angle of light exiting out from the light conductive plate thereby achieving a higher luminance. Specifically, it is considered to provide, on the bottom surface of the light conductive plate, a plurality of prisms (constituting a reflection planes) adapted to reflect light while maintaining its directivity, in place of the above-mentioned dotty roughened surface constituting an optical path conversion means.
However, when light emitted from the LEDs is incident on the light conductive plate which has the prisms formed on its bottom surface so as to extend in parallel to the entrance surface, the following problem occurs. As shown in FIG. 5, only light rays, that are emitted from LEDs and propagate in a direction perpendicular to an entrance surface 2C of a light conductive plate 2, exit out toward a viewer, resulting in remarkably poor illumination uniformity. This happens because light rays which propagate deviating from the perpendicular direction through the light conductive plate 12 are incident slantingly on the prisms, and therefore their light paths are diverted from a viewer's eye (a direction perpendicular to the exit surface), thus the light rays do not work as effective illuminating light.
A spread illuminating apparatus is disclosed in Japanese Patent Publication No. H11-287993, which is provided to address almost the same problem as mentioned above, though prisms are not used as an optical path conversion means. As shown in FIG. 6, a spread illuminating apparatus 1 disposed under an LCD 40 comprises a diffuser plate 36, a light conductive plate 2, a reflector plate 35, and a plurality of LEDs 5 disposed at an end surface 2c of the light conductive plate 2. The light conductive plate 2 has a diffraction grating 3 as an optical conversion means formed on its bottom surface 2b and has also a diffraction grating 4 formed on its top surface 2a so as to be oriented orthogonal to the diffraction grating 3.
However, a display device using a spot-like light source, such as the above-described LED, is associated with the following problems. In a color display device the light source is required to emit white light, but a practical LED emits monochromatic light. Therefore, it is commonly practiced that a fluorescent material is disposed in front of the LED emitting blue light in order to convert the blue light into white light. In the conventional spread illuminating apparatus disclosed in Japanese Patent Publication No. H11-287993, the diffraction grating is formed on the surface of the light conductive plate in order to uniformly spread light entirely over the surface of the light conductive plate. Since white light is dispersed into spectral light by the diffraction grating, the diffuser plate 36 is required which prevents the surface of the light conductive plate from getting entirely colored due to the spectral light. Also, since the light conductive plate has a grating interval of as minute as a few μm, a transcription technology with a high precision is required.